It is known that the internal combustion engine of an automotive vehicle usually includes an engine block defining at least one cylinder having a piston, and a cylinder head that closes the cylinder and cooperates with the piston to define a combustion chamber. A fuel and air mixture is disposed in the combustion chamber and ignited, resulting in hot expanding exhaust gasses causing reciprocal movements of the piston which rotate a crankshaft.
The fuel is generally provided by at least one fuel injector which may be located inside the combustion chamber. The fuel injector receives the fuel from a fuel rail, which is in fluid communication with a high pressure fuel pump that increases the pressure of the fuel received from a fuel source. The fuel injector is connected to an electronic control unit (ECU) which is configured to determine the fuel quantity to be injected inside the combustion chamber during each engine cycle and to operate the fuel injector accordingly.
To reduce polluting emissions and combustion noises, the fuel quantity to be injected in the combustion chamber is conventionally split into a plurality of injections, according to a multiple fuel injection pattern. Typical multiple injection patterns include preliminary injections, known as pilot injections, followed by one or more main injections, after and post injections.
Pilot injections are provided for injecting small quantities of fuel before a main injection, in order to reduce the explosiveness of the main injection which reduces vibration and optimizes fuel consumption.
To perform each pilot injection, the electronic control unit energizes the fuel injector for a given time, conventionally referred as energizing time (ET), thereby causing the fuel injector to open and a proportional quantity of fuel to be injected into the combustion chamber. The value of this energizing time is usually predetermined during an experimental activity performed on a test bench, and then stored as a calibration parameter in a memory system of the electronic control unit.
A drawback of this approach is that the value of the energizing time stored inside the memory system is predetermined using a nominal fuel injection system, whereas the operation of the real fuel injection system of an internal combustion engine is generally affected by both production spread and aging of its components, particularly of the fuel injectors. Therefore, the fuel quantity actually injected by a fuel injector in response to the nominal value of the energizing time generally drifts from the expected quantity, thereby causing combustion noise increase and worse exhaust emissions.
More specifically, if a pilot injection actually injects a fuel quantity that is less than the expected quantity, an engine noise increase usually occurs. On the contrary, if a pilot injection actually injects a fuel quantity that is greater than the expected quantity, the engine produces an increased quantity of particulate material. If a pilot injection misfires, besides the noise increase that inevitably occurs, NOx emissions may also be increased.
In order to prevent these disadvantages, fuel compensation strategies are implemented by the electronic control unit. These compensation strategies conventionally provide for periodically testing each fuel injector during its lifetime, in order to learn the fuel quantity drift. The learned fuel quantity drift is then used to adjust the injector energizing time in order to have repetitive performance and increased accuracy in the fuel injected quantity along the life of the injector.
These tests are performed when the internal combustion engine is running under a fuel cut-off condition, namely a condition where the electronic control unit cuts the fuel supply off, for example when the automotive vehicle is moving and the driver releases the pressure on the accelerator pedal.
During these fuel cut-off conditions, a known learning strategy provides for the electronic control unit to set the pressure within the fuel rail at a predetermined value, and then to command the fuel injector to perform a test injection in one of the cylinder of the engine, while the other fuel injectors are kept de-energized. This test injection is performed by energizing the fuel injector for the nominal value of the energizing time that should correspond to a target value of the fuel injected quantity, particularly a small fuel quantity (e.g., 1 mm3 of fuel). The combustion of this small quantity of fuel causes a variation of the engine torque, which is not perceived by the driver but can be measured with a sensor, for example with a crankshaft speed sensor. Since the engine torque is proportional to the fuel quantity actually injected during the test injection, its measured value is compared with a predetermined reference value that quantifies the engine torque that would be measured, if the fuel injected quantity were equal to the target fuel quantity. If the measured value of the engine torque is different from the reference value thereof, the electronic control unit repeats the test one or more times using different values of the energizing time, until an actual value of the energizing time is found that actually produces the reference value of the engine torque. The difference between the actual value of the energizing time and the nominal value thereof is then calculated and stored in a memory system, so that it can be retrieved by the electronic control unit and used to correct the energizing time of the pilot injections during the normal operation of the internal combustion engine.
This known fuel injector learning strategy, which is generally repeated for different values of the fuel rail pressure and for each fuel injector individually, is an iterative search that can achieve a very accurate learning of the correction to be applied to the nominal value of the energizing time to compensate for the fuel quantity drift of the tested fuel injector. However, it requires an unpredictable number of iterations to determine the actual value of the energizing time, so that the fuel injector learning phase may require a very long time to be completed, which does not always comply with the strictest OBD (On Board Diagnostic) legislation requirements.
Other fuel injector learning strategies have been proposed, which are potentially faster. One of these strategies provides for performing several test injections, using different values of the energizing time, and for calculating the quantity of fuel actually injected during each of these test injections. The values of the fuel injected quantity are conventionally calculated as a function of corresponding measured values of the engine torque. The calculated values of the fuel quantity and their correspondent energizing time values are then used in an interpolation process, which provides for determining an injector characteristic function correlating the energizing time to the injected fuel quantity, and then for applying the target value of the fuel quantity to the characteristic function to determine the actual value of the energizing time.
A drawback of this alternative strategy is that the values of the energizing time used to perform the test injections are chosen so as to disperse the quantities of fuel actually injected by the fuel injector around the target fuel quantity. This means that a group of these test injections inject fuel quantities that are smaller than the target fuel quantity. Since the target fuel quantity is a small quantity (e.g., 1 mm3), the fuel quantities injected by that group of test injections are so small that their effects on the engine torque are almost negligible. As a consequence, the measurements of the engine torque generated by these test injections may be heavily affect by noises that disturb the sensor signal, including electrical and mechanical noises, so that the following calculation of the injected fuel quantities, determination of the characteristic function and interpolation of the actual value of the energizing time may be unreliable.